Implantable microstimulator

ABSTRACT

An addressable, implantable microstimulator is substantially encapsulated within a hermetically-sealed housing inert to body fluids, and of a size and shape capable of implantation in a living body, by expulsion through a hypodermic needle. Power and information for operating the microstimulator is received through a modulated, alternating magnetic field in which a coil is adapted to function as the secondary winding of a transformer. Electrical energy is stored in capacitor means and is released into the living body by controlled, stimulating pulses which pass through body fluids and tissue between the exposed electrodes of the microstimulator. Detection and decoding means within the microstimulator are provided for controlling the stimulating pulses in accordance with the modulation of the received, alternating magnetic field. Means for controllably recharging the capacitor is provided.

This invention was made with Government support under Contract No.N01-NS-9-2327, awarded by the National Institutes of Health. TheGovernment has certain rights in this invention.

This invention relates to a microstimulator for implantation in a livingbody, in the immediate vicinity of tissue, fluids or other body cellswhich are to be electrically stimulated. It is of a size and shapecapable of implantation by expulsion through the lumen of a hypodermicneedle. It is substantially encapsulated within a hermetically-sealedhousing which is inert to body fluids and provides exposed electrodesfor electrically stimulating the desired body cells, whether muscle,nerve, receptor, gland or other area or organ of the body.

This application relates to three other patent applications filed on orabout the same date as this application, entitled STRUCTURE AND METHODOF MANUFACTURE OF AN IMPLANTABLE MICROSTIMULATOR, invented by the sameinventors as herein, IMPLANTABLE DEVICE HAVING AN ELECTROLYTIC STORAGEELECTRODE invented by one of the inventors herein, Gerald E. Loeb, andIMPLANTABLE MINIATURE COMMUNICATOR AND SENSOR MEANS invented by two ofthe inventors herein, Joseph H. Schulman and Gerald E. Loeb.

BACKGROUND

Neurological disorders are often caused by neural impulses failing toreach their natural destination in otherwise functional body systems.Local nerves and muscles may function, but, for various reasons, such asinjury, stroke, or other cause, the stimulating nerve signals do notreach their natural destination. For example, paraplegics andquadraplegics have intact nerves and muscles and only lack the brain tonerve link, which stimulates the muscles into action.

Prosthetic devices have been used for some time to provide electricalstimulation to excite muscle, nerve or other cells. Such devices haveranged in size and complexity from large, bulky systems feedingelectrical pulses by conductors extending through the skin, to implantedstimulators which are controlled through high-frequency, telemetrysignals, which are modulated rf signals, such as set forth in U.S. Pat.No. 4,524,774, Apparatus and Method for the Stimulation of a HumanMuscle, invented by Jurges Hildebrandt, issued Jun. 25, 1985. The use offrequencies of 27.12 MHz and 40.6867 MHz are there mentioned. Otherdevices have comprised a centrally-implanted stimulator package sendingstimulation signals to a multitude of distant target sites.

Complications, including the possibility of infection, arise in the useof stimulators which have conductors extending through the skin. On theother hand, in the use of implanted stimulators, difficulties arise inproviding suitable, operable stimulators which are small in size and inpassing sufficient energy and control information to the stimulators,without direct connection, to satisfactorily operate them without directconnection.

The device of the invention uses a source of electrical energy,modulated by desired control information, to selectively control anddrive numerous, small stimulators disposed at various locations withinthe body. Thus, for example, a desired, progressive muscular stimulationmay be achieved through the successive or simultaneous stimulation ofnumerous stimulators, directed by a single source of information andenergy outside the body.

The appropriate, functioning design of a suitable, small stimulator, amicrostimulator, which can be easily implanted, such as by expulsionthrough a hypodermic needle, is difficult to achieve. Notwithstandingthe small size, the microstimulator must be capable of receiving andstoring sufficient energy to provide the desired stimulating pulses, butalso, may be required to respond to received control information as topulse duration, current amplitude and shape. Further, stimulators shouldachieve a "charge balancing", that is, a balancing of current flowthrough the body tissue in both directions to prevent damage to thetissue which results from continued, preponderance of current flow inone direction.

Also, in providing the "charge balancing", it must be assured that thecurrent flow in the opposite direction from the stimulation pulse doesnot cause damage to the intermediate body cells or cause undesiredstimulation. Further, the "charge balancing" must not cause anodic orcathodic deterioration of the stimulating electrodes.

SUMMARY OF THE INVENTION

This invention teaches the electrical elements of an implantable,microstimulator useful in a wide variety of applications. Others haveproposed microstimulators and have suggested constructing them, but nonehave taught all the elements set forth herein for successfulconstruction and operation of the microstimulator.

The device of the invention is a very small stimulator and can be easilyimplanted, such as by expulsion through a hypodermic needle. It isoperable to provide stimulation pulses of desired duration, desiredcurrent amplitude and desired shape. The stimulation pulses aredelivered to the body through electrodes exposed on the outer surface ofthe microstimulator. Within the microstimulator, an induction coilreceives energy from outside the body and a capacitor is used to storeelectrical energy which is released to the microstimulator's exposedelectrodes under the control of electronic control circuitry means. Thebody fluids and tissue between the exposed electrodes provide theelectrical path for the stimulating pulse. The capacitor is controllablyrecharged, using the same exposed electrodes and using the same bodyfluids and tissue path, to achieve "charge balancing". Microelectronicsare included within the microstimulator to provide the electroniccontrol circuitry for controlling the various functions.

The microstimulator of this invention receives both energy and controlinformation from a modulated, alternating magnetic field. A coil, actingas a secondary winding of a transformer, receives the alternatingmagnetic field energy which is rectified and stored on a capacitor.Regulation of the charge on the capacitor is provided. Electroniccontrol means detects and decodes the modulating information to providethe desired control. Such control includes validating the receivedinformation, providing the stimulation signal, (its duration, amplitudeand shape), and controlling the recharge of the capacitor for thestimulating charge.

It is, therefore, an object of this invention to provide amicrostimulator of a size and shape capable of implantation by expulsionthrough a hypodermic needle.

It is another object of this invention to provide a microstimulatorwhich receives and utilizes an alternating magnetic field as a source ofpower.

Another object of this invention is to provide a microstimulator whichreceives a modulated, alternating magnetic field and detects themodulation to control a stimulating pulse.

A further object of this invention is to provide a microstimulator whicheffectively detects and demodulates the received, control signals.

Still another object of this invention is to provide a microstimulatorin which the stimulating pulse duration, pulse amplitude and pulse shapeare controlled by a received alternating magnetic field.

A final object of this invention is to provide a microstimulator inwhich energy is controllably stored in capacitor means, controllablydischarged as a stimulating pulse and which is controllably recharged toprovide "charge balancing" and to prevent undesired stimulation anddamage to the body.

DESCRIPTION OF THE DRAWINGS

Further objects and features will be apparent from the followingdescription and drawings in which:

FIG. 1 is an overall view of the device of the invention as applied toan arm, showing the primary coil in cross-section and a representationof a number of microstimulators implanted in the arm;

FIG. 2 is block diagram illustrating the transcutaneous transmission ofpower and information to implanted microstimulators;

FIG. 3 is a simplified embodiment of the electrical circuit, includingthe electronic control means, of an implanted microstimulator;

FIG. 4 is a block diagram of the electronic control means of theimplanted microstimulator;

FIG. 5 is a block diagram showing the flow of data and clock informationin a slightly different embodiment than FIG. 4;

FIG. 6 illustrates the encoding of "1's" and "0's" in the transmissionof control information, within the alternating magnetic fieldtransmitted to the microstimulator;

FIG. 7 illustrates one manner of encoding the control information withina frame of date transmitted to the microstimulator;

FIG. 8 is a cross-section side view of a microstimulator;

FIG. 9 is a top view of a microstimulator with the housing incross-section;

FIG. 10 shows a driving circuit comprised of a modulated 2 mHzoscillator, a transmitter coil and an implanted receiver coil;

FIG. 11 is an alternate embodiment for controlling the amount of energyreceived and stored by the microstimulator;

FIG. 12 is an alternate embodiment showing multiple electrodes andstorage capacitors in one end of the microstimulator;

FIG. 13 illustrates the electrical control of multiple electrodes andindividual storage capacitors; and

FIG. 14 is an alternate embodiment having an iridium electrode at eachend of the microstimulator and an electrolytic capacitor disposed withinthe housing of the microstimulator.

DETAILED DESCRIPTION OF THE INVENTION

The microstimulator of the invention is on the order of 2 mm in diameterand 10 mm long. Because of such diminutive character, it is readilyimplanted in a living person or animal through the lumen of the needleof a hypodermic syringe. The technique for implanting themicrostimulator is to inject the point of the needle of the hypodermicsyringe to the desired location for the microstimulator and to withdrawthe needle while compressing the plunger of the syringe, to expel themicrostimulator and leave it in place as the syringe needle iswithdrawn.

Because of the small size of the microstimulator, it has been difficultto establish its parameters in order to obtain the desired operatingcharacteristics. The following description sets forth suitable, workingparameters for constructing and operating such a microstimulator.

FIG. 1 shows, figuratively, how a primary coil 1, which produces analternating magnetic field, at a frequency, say, of 2 mHz, is disposedwith respect to a number of microstimulators such as 2, 3, and 4,implanted, say, in an arm 5. The microstimulators, of course, may beplanted in or near any part of the body, in the brain, a muscle, nerve,organ or other body area. The system operates as an air-gap transformerin which coil 1 is the primary winding, exterior to the body, and themicrostimulators such as 2, 3 and 4 each have coils within them whichact as secondary windings of the transformer.

Coil 1, for example, may be 12 to 20 turns of #200/38 Litz wire, andwound 20 cm long and 9 cm in diameter for operation with, for example,256 microstimulators implanted in an arm. The current flowing in suchcoil 1 may be from less than an ampere to several amperes. A preferredembodiment uses less than 1 ampere. If the transmitting coil 1 and itsassociated capacitors, not shown, in FIG. 10, are tuned to resonance atthe alternating frequency, little power would be lost in thetransmission process.

Each microstimulator has its own identifying address and, therefore, isindividually addressable. The number of microstimulators which may beactuated is therefore determined, as a practical matter, by the numberof address bits available for distinguishing the individualmicrostimulators. Eight address bits would provide 256 differentaddresses for a similar number of microstimulators. Of course, aplurality of microstimulators may be assigned the same address inapplications where simultaneous stimulation by more than one stimulatorwould be desired.

Alternatively, in FIG. 1, coil 1 may be a pancake type coil or asaddle-type coil and disposed on the surface of the skin and notnecessarily entirely encompass a limb or other body part. It may not, insuch case, be as efficient in transferring energy to themicrostimulators.

FIG. 2 is a block diagram illustrating the transcutaneous transmissionof power and information to implanted microstimulators. It shows amodulated, power source on the left, the skin and two implantedmicrostimulators on the right. Coil 1 is driven by a modulatedoscillator 6 which, in turn, is driven by a stimulation controller 7.Underneath (shown to the right of) skin 8 are implanted microstimulatorssuch as 9 and 10. Microstimulator 9 is shown in greater detail.Secondary coil 11, within microstimulator 9 receives energy and controlinformation from the modulated, alternating magnetic field provided bycoil 1 and passes such energy and information to electronic controlmeans which comprises power supply and data detector 12 which, in turn,provides power to an electrode recharge current controller 13 andstimulating electrodes 14 and 15. The recharge current, or "currentbalancing", may be on the order of 1 per cent of the stimulating currentin order to avoid undesired stimulation, damage to the body andelectrode deterioration.

FIG. 2 shows secondary coil 11 at or near the surface of the skin. Suchis for illustration only. The microstimulator may be much deeper at anydesired location within the arm, along the length of the transmittingcoil 1, FIG. 1 and, even, for some distance beyond the ends of thecoil 1. In one experimental determination, it was found that themicrostimulators may lie as far as about 5 cm. outside the volumeencompassed by coil 1.

Electrode 14, in the preferred embodiment, comprises an iridium ballhaving a stem extending into the microstimulator. The iridium ball andstem are formed by melting a 0.006" or 0.010" iridium wire such that itforms a ball at the end of the wire. A substantial portion of theiridium ball is exposed outside the stimulator and is activated. SeeFIG. 8.

Electrode 15, in one embodiment, would be placed at the opposite end ofthe microstimulator from electrode 14, FIG. 8, and is comprised ofanodized, sintered tantalum, and has a stem extending into themicrostimulator. A substantial portion of the tantalum electrode wouldalso be exposed outside the stimulator.

Electrode 15 is sintered, anodized tantalum which allows intimaterelationship with the body fluids, but is of sufficiently small cellularstructure that fibrous growth does not occur within the cells. Suchtantalum electrode 15 and the counterelectrode of iridium 14 provides,by their structure, an electrolytic capacitor, shown as capacitor 20,with resistance 21 illustrating the resistance of the path through thebody, approximately 300 ohms, between the electrodes. The capacitanceprovided by electrolytic capacitor 20 is significant, being on the orderof 2 to 30 microfarads. It has been found by others that anodizedtantalum has a very low DC leakage level when biased up to 80% of itsanodization voltage and tends to self-heal in body fluids.

In a preferred embodiment, the electrode 15 is not tantalum, but is aniridium spherical electrode 88, as shown in FIG. 14, similar to iridiumelectrode 14. In still other embodiments such electrode 88 may be merelya pellet or wire of pure tantalum, iridium, platinum, platinized metal,or suitable biomedical alloys of such metals. The capacitance is thenprovided by an internal capacitor in series circuit such electrode, asshown by electrolytic capacitor 71 in FIG. 3 and a capacitor orcapacitors such as shown at 82 in FIG. 12 which will be discussedhereafter. Such discrete capacitor, however, occupies a substantialamount of space within the microstimulator, in order to achieve thedesired amount of capacitance. The capacitor is preferably notconstructed within the electronic circuitry chip 22, but is, rather, aseparate capacitor, as shown and discussed hereafter in connection withFIG. 14.

The electrodes 14 and 15 may be constructed of other suitable metals.Platinum or platinized wire, for example, may be used. A platinum wiremay be used for electrical connection through the end of themicrostimulator, with the housing fused, or otherwise sealed, thereto. Aglass bead may be used on the platinum wire, to aid in sealing it to thehousing, as described hereinafter with respect to the tantalum stem. SeeFIG. 8.

Further, electrodes of other shape, size and disposition may be used.Elongated electrodes, with or without anchors to hold them in place maybe used. Two or more electrode wires may exit at one end, if disposedelectrically independent of each other. Such disposition may be done byusing a glass bead having two holes therethrough, much the same as abutton, and threading a wire through each hole and fusing the glass tothe wire and, subsequently fusing the glass to the housing. Such isfurther described in connection with FIG. 12.

The power supply portion of 12 provides voltage at two levels, forexample, V+, 8 to 15 volts, unregulated, for providing stimulating pulseenergy storage on the tantalum electrode and VL-, -2 to -4 volts,regulated, for power for digital logic 16. Such low voltage may also beachieved, by a tap on coil 11. A V+ voltage of 10 volts has been foundto be suitable.

Data detector 12 also provides clock and digital data information tologic 16 which decodes the control information contained within themodulated, alternating magnetic field. Such decoded information is usedby the logic 16 to control switch 17 which controls the charge build-upbetween electrode 14 and electrode 15. Logic 16, which is preferablyhigh speed, low current, silicon-gate CMOS, controls switch 18 (whichmay be a transistor, as shown in FIG. 3) which permits the stimulatingpulse current to flow between electrodes 14 and 15. Logic 16 alsocontrols current amplitude buffer 19. This controls the amount ofcurrent allowed to flow in each pulse.

Thus, all of the elements to receive, detect and store modulated,electrical energy and to decode and use the modulating information tocause the desired stimulating pulses, is provided by themicrostimulator. Such elements are all within the microstimulator exceptfor the exposed electrodes and, in the preferred embodiment, the storagecapacitor for the stimulating pulses. Such storage capacitor 20, FIG. 2,is provided by tantalum electrode 15 together with iridium counterelectrode 14, extending beyond the microstimulator and thus immersed inbody fluids.

Such electrode 15, although it may only be 1.5 mm on a side, will easilystore 100 microcoulombs of charge. Only 3.84 microcoulombs of charge isrequired for a 15 ma stimulating pulse having a 256 microsecondduration. Furthermore, the charge may be stored at a voltage (say, 10volts) sufficient to overcome the output impedance (approximately 300ohms) of the two electrodes and the intervening tissues of the body.Upon the largest stimulation pulse, the voltage between the electrodes14 and 15 may drop to 8 volts, for example.

FIG. 3 is a simplified embodiment of the electrical circuit of themicrostimulator. It shows power rectification and signal detection inmore detail than that shown in FIG. 2. Most of the electrical circuit ofthe microstimulator is contained on an integrated circuit, ormicrocircuit, chip 22.

The coil 11 is tuned by capacitor 23 to the frequency of the alternatingmagnetic field. In some instances, capacitor 23 may be provided by thestray capacitance of coil 11. Resistor 67 and Schottky diode 26 providerectification and a power bus 69 for the positive side of the receivedelectrical energy. If it is desired, an external diode, such as thatshown at 26A may be utilized. It is connected around microcircuit chip22, from one end of coil 11 to the external connection of the electrode15. This external diode 26A is particularly useful in the event the chipdiode 26 fails or does not meet the product specification or wouldotherwise prevent the electronic chip 22 from being usable oracceptable. The physical disposition of such diode 26A is better shownin FIG. 14. In FIG. 3, capacitor 24 serves to smooth out the ripple inthe power bus 69. Shunt regulator 25 serves as a current shunt toprevent the voltage between the positive and negative power busses 69and 70 (and thus between the electrodes 15 and 14) from becoming toohigh, say, above 15 volts. Shunt regulator 25 may be comprised of one ormore Zener diodes and resistors or more sophisticated voltage regulatingcircuitry. The shunt regulator 25 effectively controls the excess energywhich is received by dissipating it at an acceptable rate. It isexpected that dissipation would not exceed approximately 4milliwatts/cm², which is about 20% of levels which have been foundacceptable in cardiac pacemaker dissipation.

Another method of controlling the amount of energy received and storedby the microstimulator would be by connecting a voltage regulator inplace of regulator 25, to switch a capacitor in and out of circuit inparallel with capacitor 23. FIG. 11, discussed hereafter, illustratessuch a voltage regulator 80 connected across voltage bus lines 69 and 70and controlling transistor 79 which switches capacitor 78 in and out ofthe receiving circuit comprising coil 11. In this manner, the amount ofelectrical energy stored in the capacitor means is controlled.

It is pointed out that lowering the Q of the power supply and thereceiving circuit, by a shunt-regulator which dissipates energy orprovides a current-sinking path, effectively stabilizes the electroniccontrol circuit, particularly the demodulator and detector so thatvariations in loading do not interfere with signal demodulation ordetection. At the same time, the shunt-regulator, or current-sinkingmeans, prevents overcharging or overloading the storage capacitor meansin the microstimulator.

Level shift 33 is connected to receive the energy received by thereceiving coil 11 and drops the peaks to a detection range so the peakdetector 29 can detect the peaks. From that detected signal, a shortterm detected signal is obtained by capacitor 27 and resistor 28 and along term average detected signal is obtained by capacitor 32 andresistor 31 (which have a longer time constant than the first resistorand capacitor). The short term detected signal and the long term averagedetected signal are fed into comparator 30 which provides the detecteddata to be processed by the logic 16. Such logic controls thestimulation transistor 18 and the recharge transistor 68. Whentransistor 18 is conducting, transistor 68 is non-conducting and thecharge between electrodes 14 and 15 is used to provide a stimulatingpulse. In the preferred embodiment only a small part of the chargebetween the electrodes is utilized in the stimulating pulse.

Logic 16, would not require the full voltage of the V+ between lines 69and 70, and may be operated on 2 to 4 volts, by a series regulator, (notshown) which would reduce and control the supply voltage to logic 16.

In order to restore the full charge between electrodes 14 and 15, or, inother words, the charge on the capacitor 20, transistor 18 is controlledto be non-conducting and transistor 68 is controlled to be conductingand the voltage busses 69 and 70 charge up the electrodes.

If the microstimulator does not use anodized, porous tantalum or otherstructure which provides an electrolytic capacitor when disposed in thebody fluids, then a miniature capacitor may be required to be disposedinside the housing of the microstimulator. Such a capacitor is shown ascapacitor 71 or as capacitor 82 in FIG. 12. Such capacitor may bemanufactured on the electronic chip 22, but is preferably external tothe electronic chip 22, as shown by capacitor 82, one of three, forexample, in FIG. 12. An electrolytic capacitor 82, having 10 or 15microfarads, would be typical.

FIG. 4 is a block diagram illustrating one implementation of thecircuitry of the electronic control means of the microstimulator.Assuming a 2 mHz, modulated, alternating magnetic field is transmittedfrom outside the skin, coil 11 and capacitor 23 provide the signal atthat frequency to data detector 12A, (part of power supply and datadetector 12, FIG. 2). Assuming that the modulating information iscontained in 36-bit frames, data detector 12A provides such 36-bit framedata to data decoder 34.

Data decoder 34 sends the data, to 15-bit sparse addressable latch 37and the frame/address detector 38. Latch 37 is essentially a CMOS RAMstorage device which stores only a portion of the received frame, inthis instance, pulse duration and pulse amplitude. Location within aframe of data is signaled by the 8-bit counter 35 on line 42 to latch37, frame/address detector 38 and comparator 39.

Latch 37 captures only those 15 bits relating to pulse width, pulseamplitude, range, recharge level and shape, (pulse tail or not), withina 36-bit frame, which is discussed in more detail hereafter. The latch37 may be much like a CMOS static RAM with 6 transistors per storagecell.

Frame/address detector 38 looks at an incoming frame bit by bit anddetermines whether the information is addressed to this microstimulator.It also checks the validity of the entire frame, which may beparity-encoded to insure accuracy. In the preferred embodiment,Manchester encoding of the bit transmission is also used.

A frequency source (not shown) provides one or the other of two clocksignals to 8-bit up counter 35, under the control of mode control 36.Such clock signals may be asynchronous with the 2 mHz frequency receivedfrom outside the skin. In the preferred embodiment, however, the clocksignals are synchronous with the frequency of the received alternatingmagnetic field, by being derived from the received signal. This would beaccomplished by clock signals which are obtained synchronously from thedata decoder 34, from the alternating magnetic field frequency. As shownin FIG. 4, the asynchronous version, one clock signal is a 1 microsecondper count clock signal used in controlling the duration of thestimulating pulse and the other is an 8 microseconds per count clocksignal used in developing and validating the data being received. Thus,the mode control 36 calls for one or the other of two modes, one mode,"generate pulse" (the 1 microsecond per count mode) and the other mode,"search for valid frame", (the 8 microsecond per count mode). Modecontrol 36, by controlling line 41, controls the input to 8-bit counter35 and determines which count mode, 1 microsecond per count or 8microseconds per count, is being received by the counter 35 from thefrequency source.

During the 8 microsecond mode, if a valid 36-bit frame is not receivedby frame/address detector 38, or the address is not to this stimulator,the detector 38 resets itself, notifies mode control 36 which resets the8-bit counter to zero and the search for a valid frame having a correctaddress commences again. If a valid 36-bit frame is received by detector38, it notifies mode control 40 which switches to "generate pulse" mode,by clearing the 8-bit counter 35 and switching counter 35 to receive the1 microsecond per count. The output driver 40 controls transistor 18which is turned on to allow a stimulating pulse for the requisite timeas determined when comparator 39 determines that the count, from 8-bitcounter 35, is equal to the stored count in latch 37. When such countsare equal, comparator 39 advises mode control 36 (that the pulse hasbeen on the required time) and to stop. Mode control 36 then stopsdriver 40 which turns off transistor 18, so that it is non-conducting.While transistor 18 is turned on, of course, tantalum electrode 15 andiridium electrode 14 are discharging a portion of the electrical chargebetween them, on capacitor 20, FIG. 3, thus providing a stimulatingpulse through the body.

Transistor 68 is controlled by output driver 40 to restore the fullcharge on tantalum electrode 15 with respect to iridium electrode 14, inpreparation for the next stimulating pulse. The recharge current is 100microamps, in high recharge, and 10 microamps, in low recharge. Thestimulating pulse amplitude may be, for example, 2 to 30 ma, in the highstimulation range, and 0.1 to 1.5 ma in the low stimulation range.

FIG. 5 is another embodiment of the microstimulator, showing more detailas to the flow of data and clock information, than is shown in FIG. 4.The induced modulated, alternating magnetic field is received by coil 11and capacitor 23 and is transmitted, as discussed with reference to andillustrated in FIG. 2, to power supply and detector 12, which provides a2 mHz clock (from the received 2 mHz signal) and the raw data to datadecoder 34, inside logic 16, shown in dotted lines. Both the powersupply and data detector 12 and the logic 16 are included on the sameintegrated circuit chip 22 discussed hereafter in connection with FIGS.8 and 9.

Data decoder 34 develops a 1 microsecond per count and an 8 microsecondper count clock signals and send them to mode control 36. Data decoder34 also sends the data to 15-bit RAM 37 and frame/address detector 38.The data written into 15-bit RAM is controlled by the mode controller37, (after frame/address detector 38 has validated the frame of data tocontroller 37), which allows 15-bit RAM to store the data bitspertaining to stimulating pulse width, amplitude, pulse range, rechargelevel, and whether a pulse tail is to be provided.

Ram 37 stores received data directing the amplitude of current in thestimulating pulses. In a preferred embodiment, two ranges of current areselectable, one range, 0.1 to 1.5 mA, and the other range, 2 to 30 mA.Ram 37 sends the range signal to output driver 40, together with signalsas to amplitude within such range, the recharge level (high and lowlevels of recharge are allowed, say, 100 microamperes and 10microamperes, respectively), and whether a pulse tail is desired on thestimulating pulse. Driver 40 carries out such information by control ofstimulating transistor switch 18 and recharge transistor switch 68.

A pulse tail, an exponential tail, (or a "ramp", as termed in FIG. 7) ona stimulating pulse provides a capability of controlling the directionof stimulation pulse flow along a nerve. Work at Case Western Reserve byFang and Mortimer has determined that a pulse-tail will achieve anodalblock of large diameter axons in nerve cuff applications. A tail of 300microseconds is suggested by that work.

As in FIG. 4, frame/address decoder 38, validates or invalidates thereceived frame of data and notifies mode control 36. If the frame isinvalid, both the counter 35 and detector 38 are reset by mode control36 to commence again.

As in FIG. 4, comparator 39 compares the count from 8-bit counter 35 and15-bit RAM 37 and indicates to mode control 36 when the stimulatingpulse duration (width) is equal to that ordered by the received datastored in RAM 37. Upon it becoming equal, mode control 36 ceasesadvising output driver 40 to stimulate.

A particular feature is desirably included in detector 12. If thereceived power drops to less than 5 volts, or if there is no carrier orloss of carrier, the detector 12 shuts down all parts of the system by areset signal. Also, mode control 36 will not send out any signal to theoutput driver 40 to generate a stimulating pulse, until the receivedaddress and control information have been validated. Thus, undesiredstimulation does not take place during initial charge-up of the storagecapacitor means, or during intervals of low or incorrect inducedvoltages or control signals.

Mode control 36 is also preprogrammed to provide a predetermined,initial, pull-up of the CMOS circuitry (by the precharge signal to theframe/address detector 38). Implementing the CMOS with an initialpull-up and then pull-down upon receipt of validated signals, simplifiesthe logic.

Further, such mode control 36 maintains control and does not directstimulation until successful, valid address bits and control bits arereceived and so indicated by the frame/address detector 38.

FIG. 6 shows one method of using amplitude modulation of the oscillator6, FIG. 2, to carry the control information being sent to themicrostimulator from outside the skin. The encoding uses 16 cycles ofthe 2 mHz carrier, at its full, received amplitude, as a logical "0" and8 cycles at the full, received amplitude plus 8 cycles of reducedamplitude, as a logical "1", as illustrated in FIG. 6. FIG. 6illustrates the reduced amplitude as being 0.8 the height of the fullamplitude of the received wave. Waveform 44 represents a logical "1" andwaveform 45 represents a logical "0".

In FIG. 6, it may be seen that 16 cycles of the 2 mHz amplitudemodulated carrier would provide an 8 microsecond wide bit Clock pulsesof 1 microsecond (2 cycles of the 2 mHz carrier) and 8 microseconds (16cycles of the 2 mHz carrier) are readily generated for timing within themicrostimulator as discussed hereinafter.

FIG. 7 illustrates one manner of encoding a frame of data, of "1's" and"0's", within the alternating magnetic wave, to carry the controlinformation. Using an information frame 36-bits long, the information isencoded and transmitted to the microstimulator.

As shown in FIG. 7, the first six bits 46 comprises five "0's" followedby a "1" to indicate the start of the frame. Therefore, the first sixbits 46 indicates "start" or "get ready". The next eight bits 47 containthe address of the device being addressed. Consequently, 256 devices maybe individually addressed. The next eight bits 48 contain informationpertaining to the pulse width, say, from 1 to 256 microseconds. The lastset of eight bits 49 contains further information to control the pulse.In the example shown, four of the last eight bits, C₇₋₄ contain thecurrent amplitude values (say, 0-15 steps). The next bit, C₃, containsthe range of current amplitude (say, 0.1 to 1.5 ma or 2 to 30 ma). Thenext bit, C₂, whether the stimulating pulse is square or has a pulsetail (called a "ramp" in the drawing). The next bit, C₁, sets thecurrent recharge value (10 or 100 microamperes), and the final bit, C₀,is encoded as a parity bit, for error checking.

Thus, the stimulating pulse is controlled as to when it should exist,its duration (width), its range of amplitude and specific amplitude inthat range, its shape, and the recharge current to get ready for thenext stimulating pulse.

Idle pulses of "0's" followed by "1's" may be sent as shown in FIG. 7,before and after each frame of data.

Manchester encoding may be used to assure reliability. In such encoding,there is always a transition, from high to low or vice versa, at the endof sixteen cycles, which is the bit time. See FIG. 6. If there is atransition at 8 cycles, the state time, the bit may be termed a "1". Ifthere is no transition at 8 cycles, the bit is then a "0".

Control of additional microstimulator capabilities may be added byextending the information frame of data to more than 36 bits.

FIG. 8 is a cross-section side view of a microstimulator. The iridiumelectrode 14 and the tantalum electrode 15 are at opposite ends of themicrostimulator. The anodized layer 15A on tantalum electrode 15 and theactivated layer 14A on iridium electrode 14 provide a substantialcapacitance between them when the microstimulator is implanted and theporous tantalum and, thus, the iridium ball are immersed in body fluids.

In the manufacturing process, the exposed surface of electrode 14 isactivated by immersing it in a phosphate-buffered saline solution andtouching its outside surface with a whisker probe, (fine iridium wire of0.003" D), and cycling for 20 to 30minutes at 0.5 volts per second to amaximum of plus or minus 0.8 volts. The cyclic voltammetry builds up anelectrically conductive layer of iridium hydrous oxide, layer 14A, (anactivated layer), that is capable of being cycled reversibly between the+3 and +4 valence states, thereby transforming electron motion in theunderlying metal into ion fluxes in the surrounding solution withoutinducing irreversible electrolysis of the water or metal. Theinterfacial impedance tends to be very low, also, reducing the necessaryvoltage between the electrodes 14 and 15 to be used in obtainingstimulation.

The coil 11 is shown wound around a ferrite core 50. Such core iscylindrical and is manufactured in two halves with a U-channel in eachone. When such halves are placed together, a channel is thus formedbetween them. The iridium stem 52 passes through such channel.Integrated circuit chip 22 is carried on a ferrite shelf 73 and isconnected to receive the output of the coil 11. Iridium electrode 14 isconnected through its stem 52 and wire 53 to receive the output of chip22 and be controlled thereby. Tantalum electrode 15 is connected throughits stem 54 and through weld shim 55 to a metallized pad 56 on the shelf73 of ferrite 50. Integrated circuit chip 22 sits on metallized pad 56and is thus connected through its base, or substrate, to stem 54 ofelectrode 15. Thus, electrodes 14 and 15 are both connected to receivethe output of chip 22 and be controlled thereby. The integrated circuitchip 22, in a preferred embodiment, is double-poly, p-well (3 micron)CMOS in which the substrate is at the V+ supply rail to which thetantalum electrode may be readily connected.

In another embodiment, weld shim 55 and metallized pad 56 are replacedby a small metallized pad on ferrite shelf 73 to which both the tantalumstem 54 and integrated circuit chip 22 are electrically connected, byflying bond wires or other means.

It is noted that such electrodes, or their leads, are hermeticallysealed to housing 72. The preferred embodiment comprises a housing ofN51A glass or other suitable biomedical grade capillary tubing having aninner diameter of about 1.25 mm's. It is available from or through glassfabrication houses such as Kimbel Glass, Corning Glass and others.

It is important to select a glass which is stable in body fluids andwhich matches pretty well the coefficient of thermal expansion of thetantalum and iridium because of the heating operations involved infusing the electrodes to the glass housing.

In the case of the tantalum electrode 15, the stem 54 is first fused toglass bead, or washer, 74, and then the glass bead 74 is fused to thehousing 72. In the case of the iridium electrode 14, the ball portionprovides a good sealing surface and is fused directly to the housing 72.In alternate embodiments, a simple platinum or titanium wire, or wire ofother suitable metal, may pass through the end of such microstimulatorand be fused to the housing 72 to provide a hermetic seal. Externalelectrodes may be constructed on otherwise attached to such wires, asdesired, or such wires may, themselves, be the electrodes.

FIG. 9 is a top view of a microstimulator with only the housing 72 shownin cross-section. Coil 11 is illustrated as being numerous turns of afine wire and is included within the microstimulator housing 72 which isonly approximately 10 mm long. The coil 11 and ferrite core 50 occupy alarge part of the housing 72. Coil 11 may be approximately 200 turns ormore of a fine, copper wire. The coil 11 acts as the secondary of atransformer and receives energy by induction from outside the body.

A preferred embodiment in the construction of coil 11, is 250 turns of0.00102" D, or finer, insulated, copper wire on a ferrite core having adiameter of approximately, 0.050". Due to the stray, or distributedcapacitance of such windings, the coil would be resonant atapproximately 2 mHz.

In FIG. 9, the two ends of the coil 11 are shown connected to two metalpads 57 and 58, which may be made, for example, of palladium-silverdisposed on a ferrite shelf 87, shown in FIG. 8. Such pads 57 and 58, inFIG. 9, are further connected to provide input to the integrated circuitchip 22 by means of gold bonding wires 76 and 77 being connected to pads59 and 60. The integrated circuit chip 22, in turn, provides acontrolling output to the two electrodes 14 and 15, as explainedpreviously in connection with FIG. 8.

FIG. 10 shows a 2 mHz oscillator 61 receiving modulation and drivingexternal transmitter coil 1, which provides an alternating magneticfield modulated in accordance with desired information. Coil 11 isadapted to receive the alternating magnetic field through skin 8.Suitable transmitters and receivers are well-known in the art. Class Edrivers, which are highly efficient if properly modulated and operated,are particularly suited to drive coil 1. Such class E drivers arewell-known in the art and an analysis may be found in an articleentitled, "Exact Analysis of Class E tuned Power Amplifier at any Q andSwitch Duty Cycle," Kazimierczuk and Puczko, IEEE Transactions onCircuits and Systems, Vol. CAS-34, No. Feb. 2, 1987, pp. 149-159.Numerous additional references discussing class E drivers and amplifierswhich may be used as drivers are therein cited. Inductive transdermallinks are further disclosed and discussed in U.S. Pat. No. 4,679,560,for Wide Band Inductive Transdermal Power and Data Link, inventor,Douglas C. Galbraith and in "RF Powering of Millimeter- andSubmillimeter-Sized Neural Prosthetic Implants,", William J. Heetderks,IEEE Transactions on Biomedical Engineering, Vol. 35, No. 5, May 1988.

FIG. 11 illustrates another means of controlling the amount of energystored by the microstimulator. The method of controlling the amount ofenergy stored is to control the amount received by the coil 11. If it isde-tuned from the resonant frequency, by varying the capacitance incircuit with it, it will not receive so much energy. Coil 11 andcapacitor 23 form a circuit tuned to the frequency of the signal beingtransmitted by the transmitter outside the skin. Capacitor 78 andtransistor switch 79 are connected across coil 1;. Voltage regulator 80is connected to receive the rectified and smoothed voltage across buslines 69 and 70 and provides a control of switch 79. When the voltagebuild-up is too high across bus lines 69 and 70, transistor switch 79completes the circuit and capacitor 78 is switched into circuit withcoil 11. The circuit comprising coil 11 is then de-tuned from thetransmitting frequency and receives less power than when tuned to thetransmitting frequency. As soon as the bus voltage drops, voltageregulator 80 opens transistor switch 79 and the coil 11 is again in acircuit tuned to the transmitting frequency.

FIG. 12 illustrates multiple electrodes 15B, 15C, and 15D in place ofthe single tantalum electrode 15 illustrated in FIGS. 8 and 9. Suchelectrodes in FIG. 12 may be platinum, platinized wire, tantalum,iridium, or other suitable biocompatible, metal. They appear to be ofthe same metal, but of course, each electrode may be different than theothers. Also, they may be of other shapes than the simple wire shapeshown in FIG. 12. In this embodiment, the electrodes do not provide anelectrolytic capacitance by being immersed in body fluids, but, ratherare each connected in series with an axial capacitor, such as axialcapacitor 82. Such capacitors may be electrolytic, tantalum capacitors,or other capacitors readily available from manufacturers. The capacitorsare, in turn, connected to individual connection pads on electroniccircuitry chip 22 by flying bond wires such as wire 85.

In FIG. 13, it may be seen that the charge build-up on such capacitorsis controlled by control circuitry on the electronic circuitry chip 22which controls transistor switches such as switch 84 which connects thestorage capacitor of each electrode to the V+ supply. In order tocomplete the charging circuit, transistor switch 86, in circuit withelectrode 14 must be closed, connecting the counterelectrode 14 to theV- supply. The charging circuit is thus completed through the bodyfluids and tissue and a charge is stored on the axial capacitors of eachelectrode, such as capacitor 82. The electrodes 15B, 15C and 15D areshown as having individual capacitors and switching transistors, butthey may, of course, have a single capacitor and switching transistor,driving them all.

For stimulation, which discharges, or partly discharges the axialcapacitors, the transistors such as transistor 83 are closed, togetherwith transistor 86, thus connecting the electrodes 15B, 15C and 15D incircuit with counterelectrode 14 to provide stimulation pulses, providedby the charge on the axial capacitors, to the body tissue and fluids.Control of such transistor switches to control pulse timing, duration,amplitude and shape, utilizes the same concepts previously discussed inconnection with FIGS. 8 and 9, in connection with a single tantalumelectrode.

The embodiment shown in FIGS. 12 and 13 allows the provision of a singlelarge stimulating pulse, or quick, successive pulses, or spaced pulsesat rates and magnitudes not otherwise achievable. Such wire electrodesmay be longer than the proportions shown in the drawings. The techniquefor using a hypodermic syringe for implanting the electrodes in the bodypermits the expulsion of the microstimulator by withdrawing the syringeas the microstimulator is expelled, thus leaving the microstimulator andits electrodes in place. A microstimulator having a long electrode orlong electrodes could thus easily be implanted by a syringe.

Transistor 86 is not necessary in some instances and the iridiumelectrode 14 is connected directly to the V- power supply bus. However,transistor 86 can be used to effectively isolate iridium electrode 14from the power supply and other circuitry within the system.

Coil 11 is shown figuratively in FIG. 13, unconnected, but would, ofcourse, be connected as shown in FIGS. 3 and 9. Coil 11, of course, isconnected to electronic circuitry chip 22, as shown in FIG. 9, by bondwires 76 and 77. One of such wire connections, wire 76 is shown in FIG.12.

Axial capacitors need not be used, but other shapes may be utilized.Referring to FIG. 8, for example, a capacitor, or multiple capacitors,may be located in the electrical circuit and at the place occupied byweld shim 55. It may be seen that one side of the capacitor would beconnected to the tantalum electrode, as shown in FIG. 8, and the otherside, to the electronic circuitry chip 22 through metallized pad 56.

FIG. 14 illustrates a preferred embodiment in which iridium electrodes14 and 88 are disposed at opposite ends of the microsimulator. Electrodestems 52 and 89 pass through glass beads 90 and 91, respectively. Suchconstruction allows for assembling the microstimulator by threading theelectrode stems 52 and 89 through glass beads 90 and 91, fusing eachglass bead to its respective stem, then fusing the glass beads to thehousing 72, when the glass beads and the internal assembly are suitablypositioned in the housing.

Iridium electrode 88 is connected through its electrode stem 89 and awelded, soldered, or otherwise connected wire 92 to a metal plate, or tometallization 93, which is further connected to the one electrode, ofelectrolytic capacitor 71. In FIG. 14, such one electrode is the outersurface, or case, of the capacitor 71. Electrolytic capacitor 71 isfurther connected through its other electrode 95, to metal shim 96 andmetal plate 97, to the base of electronic circuitry, IC chip 22.

An additional electrical connection 100, may be made from the other sideof the electrolytic capacitor 93 to electronic chip 22, to provideelectrical energy directly from electrolytic capacitor 71 to electronicchip 22.

It has been found that in such miniature construction, ferrite shelf 73is prone to be broken. Making shelf 73 relatively short and adhering alonger metal plate 97 thereto provides sufficient area to mountelectronic chip 22, shim 96 and diode 26A. The plate may be made of mostany inert metal or metal alloy. Nickel and nickel alloy are particularlysuitable. Flying bond wires, such as are shown at 77, 53, and 98, makedesired electrical connections to the electronic chip 22.

The microstimulator may be assembled by assembling the internalstructure, with electrodes at each end, and sliding such assemblythrough the housing 72 until the assembly is in place, with an iridiumelectrode extending out each end of the housing. The glass beads 91 and52, which have previously been fused to the stems 52 and 89, are thenfused to the housing 72. The housing may be filled with an inertmaterial impervious to water, such as epoxy or silicon rubber through ahole in the housing by means of a syringe 99. The filling with epoxy,silicon rubber or other suitable inert material, may occur while theassembly is being placed within the housing or thereafter. Any holes orvents for filling the housing may be sealed up with the inert materialor sealed by other means.

Inasmuch as the electronic chip 22 is light sensitive, a light barriermust be provided. Such light barrier may be a film or mask placed on thechip, an opaque or colored material used to fill the microstimulator, ora housing which is opaque or colored so as to prevent undesired lightfrom reaching the chip 22.

Although specific embodiments and certain structural and electricalarrangements have been illustrated and described herein, it will beclear to those skilled in the art that various other modifications andembodiments may be made incorporating the spirit and scope of theunderlying inventive concepts and that the same are not limited to theparticular forms herein shown and described except insofar as determinedby the scope of the appended claims.

We claim:
 1. An implantable microstimulator substantially encapsulatedwithin a hermetically-sealed housing which is inert to body fluids, theimprovement comprising said microstimulator being of a size and shapecapable of implantation by expulsion through a hypodermic needle, saidmicrostimulator having therein a coil adapted to function as thesecondary winding of a transformer, said coil adapted to be disposed ina modulated, alternating magnetic field, at least two exposed electrodesfor providing electrical stimulation, electronic control means,capacitor means for storing power for said electrical stimulation, andfor said electronic control means, means for controllably charging saidcapacitor means from said alternating magnetic field, said electroniccontrol means controlling discharge of said capacitor means to provideelectrical stimulation through said exposed electrodes and the electricpath provided by the body and said electronic control means furtherproviding control of one or more of the duration, the current and theshape of said stimulus pulses in accordance with the modulation of saidalternating magnetic field.
 2. The microstimulator recited in claim 1wherein is included means for controlling said microstimulator in theevent of failure to receive sufficient power or invalid modulation, lackof sufficient modulation, and upon startup.
 3. The microstimulatorrecited in claim 1 wherein said capacitor means is charged by a firstelectrical circuit using the body as part of the charging path anddischarged, while said first electrical circuit is broken, by a secondelectrical circuit connecting said electrodes together inside saidmicrostimulator.
 4. The microstimulator recited in claim 1 wherein saidmeans for controllably charging said capacitor means, comprises meansfor regulating the current flow in charging said capacitor means.
 5. Themicrostimulator recited in claim 4 wherein said means for regulating thecurrent flow, regulates said current flow below that which wouldstimulate said body area, have adverse effect on said body area and haveadverse effect on said electrodes.
 6. The microstimulator recited inclaim 1 wherein said capacitor means is shunt-regulated at least in partto lower the Q of the power supply in aid of stabilization of saidelectronic control means.
 7. The microstimulator recited in claim 1wherein is included within said microstimulator, means for controllingthe amount of electrical energy stored by said microstimulator.
 8. Themicrostimulator recited in claim 7 wherein said amount of electricalenergy stored by said microstimulator is several times the amount ofelectrical energy required by said microstimulator to stimulate.
 9. Themicrostimulator recited in claim 7 wherein said means for controllingthe amount of energy stored by said microstimulator comprises means fordissipating excess energy induced into said microstimulator.
 10. Themicrostimulator recited in claim 9 wherein said means for dissipatingexcess energy comprises current-sinking means.
 11. The microstimulatorrecited in claim 7 wherein said means for controlling the amount ofenergy stored by said microstimulator comprises a shunt regulatorcontrolling the charging of said capacitor means.
 12. Themicrostimulator recited in claim 7 wherein said means for controllingthe amount of energy stored by said microstimulator comprises means forvarying the resonant frequency of the electrical circuit comprising saidcoil.
 13. The microstimulator recited in claim, 1 wherein both of saidelectrodes are activated iridium.
 14. The microstimulator recited inclaim 1 wherein is also included means for preventing stimulation pulsesupon failure of said microstimulator to receive sufficient inducedpower, for at least one of, proper stimulating pulses or controlinformation.
 15. The microstimulator recited in claim 1 wherein saidelectronic control means comprises detector means and decoding meansreceiving information from said modulated, alternating magnetic field,as to one or more of, stimulus pulse duration, stimulus pulse current,stimulus pulse shape and identifying the specific microstimulator, towhich said modulated information is sent.
 16. The microstimulatorrecited in claim 15 wherein said decoding means is synchronized with thefrequency of said alternating magnetic field.
 17. The microstimulatorrecited in claim 15 wherein said detector means comprises means forobtaining and comparing the short term signal and the long term averageof such detected signal and providing the compared signal to saiddecoding means.
 18. The microstimulator recited in claim 15 wherein isincluded means for providing control of one or more of the duration, thecurrent and the shape of said stimulus pulses in accordance with theoutput of said detection and decoding means.
 19. The microstimulatorrecited in claim 18 in which said microstimulator comprises means forproviding stimulation and recharging in two substantially differentranges of current amplitude.
 20. The microstimulator recited in claim 15wherein is included means for storing the pulse control informationreceived by said microstimulator, and where is included mode controlmeans for controlling the storing of information in said means forstoring in accordance with an evaluation of the address and validity ofthe received information.
 21. The microstimulator recited in claim 1wherein is included means to control said electrodes during the initialcharge-up of said capacitor means for storing power and until validcontrol information is received.
 22. The microstimulator recited inclaim 1 wherein said microstimulator is of a size approximately 2 mm indiameter and 10 mm in length.
 23. The microstimulator recited in claim 1wherein is included means for controlling the amount of energy inducedin said coil.
 24. The microstimulator recited in claim 23 wherein saidmeans for controlling energy induced in said coil comprises means forvarying the capacitance in circuit with said coil, for detuning saidtuned coil from said alternating magnetic field.
 25. The microstimulatorrecited in claim 1 wherein said coil comprises 250 turns of 0.00102" Dwire and said coil has a distributed capacitance which provides aresonant frequency of said coil at 2 mHz.
 26. An implantablemicrostimulator substantially encapsulated within a hermetically-sealedhousing inert to body fluids, the improvement comprising saidmicrostimulator being of a size capable of being implanted through ahypodermic needle, said microstimulator adapted to electricallystimulate living tissue, said microstimulator having means for receivinga modulated, alternating magnetic field providing power and information,stimulating circuit means comprising two electrodes and capacitor meansto store received electrical energy, said two electrodes adapted to bedisposed in one or more of body fluids and tissue and connected inseries circuit with said capacitor means for discharging said capacitorat least partially into said fluids and tissue, providing a stimulatingpulse thereto, means responsive to modulations of said received energyto control the duration of said stimulating pulses to specific valueswithin predetermined ranges and to control the current amplitude of saidstimulating pulses to within specific values within predetermined rangesand means for providing a balancing current flow, between said twoelectrodes, between said stimulating pulses, in the opposite directionfrom said stimulating pulses.
 27. The microstimulator recited in claim26, wherein is included means for controlling the shape of saidstimulating pulses.
 28. The microstimulator recited in claim 27 in whichsaid stimulation pulses are controlled to have either abrupt endings orendings with an exponential tail.
 29. The microstimulator recited inclaim 26 in which said means for providing a balancing, current flowcomprises current-regulating means, preventing excess current flow inthe direction opposite to said stimulating pulses.
 30. Themicrostimulator recited in claim 26 in which said balancing, controlledcurrent flow between said two electrodes maintains the voltage on thenegative one of said electrodes at a voltage substantially below thepotential causing cathodic depositions on said negative electrode. 31.The microstimulator recited in claim 26 in which said means forcontrolling pulse duration and said means for controlling pulse currenteach comprise further means for controlling said pulse duration andpulse current to low values as default upon one or more of startup ofthe system and lack of sufficient modulation information and lack ofadequate received energy.
 32. The microstimulator recited in claim 26 inwhich is included means for controlling said microstimulator upon one ormore of lack of sufficient modulation of said alternating magnetic fieldand errors in said modulation.
 33. An implantable microstimulatorsubstantially encapsulated within a hermetically-sealed housing which isinert to body fluids, the improvement comprising said microstimulatorbeing of a size and shape capable of implantation by expulsion through ahypodermic needle, said microstimulator having a coil for receiving analternating magnetic field, stimulating electrode means at one end and acounter electrode at the other end, capacitor means in circuit with saidstimulating electrode means and means for controllably charging saidcapacitor means from voltage induced in said coil, means for at leastpartly discharging said capacitor means through said electrode means andsaid counter electrode into said body and means for controllablyrecharging said capacitor means.
 34. The microstimulator recited inclaim 33 wherein said electrode means comprises a plurality ofelectrodes and said capacitor means comprises an plurality ofcapacitors, each disposed in circuit with a respective one of saidplurality of electrodes and said means for controllably charging saidcapacitor means comprises means for individually charging saidcapacitors.
 35. The microstimulator recited in claim 34 wherein saidcapacitors in circuit with said electrodes may be at least partlydischarged, individually.
 36. An implantable microstimulator having ahousing of a size capable of expulsion through the lumen of a hypodermicneedle, said microstimulator having inert, metallic electrode meanscomprising at least two electrodes electrically connected to the insideof said housing through a hermetic seal, one electrode being disposed ator near one end of said housing and the other electrode at or near theother end of said housing, electronic control circuitry and electrolyticcapacitor means disposed within said housing and connected between oneof said electrodes and said electronic control circuitry, inductionmeans comprising a coil for receiving a modulated, alternating magneticfield to provide power and control information for said microstimulator,said electronic control circuitry connected in circuit with said coiland said second electrode and providing control and electrical energyfor the charge of said capacitor means and, at least partial discharge,of said capacitor means through said electrode means.
 37. Themicrostimulator recited in claim 36 wherein said first electrode meanscomprises a plurality of electrodes.
 38. The microstimulator recited inclaim 37 wherein said capacitor means comprises a capacitor in serieswith each of said electrodes of said plurality of electrodes of saidfirst electrode means.
 39. The microstimulator recited in claim 38wherein said electronic control circuitry comprises switches andregulator means for controlling the charge and at least partialdischarge of electrical energy on said plurality of capacitors.
 40. Themicrostimulator recited in claim 36 wherein said metallic electrodemeans comprises an iridium electrode disposed at each end of saidmicrostimulator.